1. Technical Field
The present invention relates to a polarization separating element which is generally called a wire grid polarization element.
2. Related Art
A wire grid polarization element has a configuration in that a grid formed of metal is laid on a transparent substrate. The biggest property of the wire grid polarization element is that the pitch of the grid is sufficiently shorter than a wavelength being used.
Since the wire grid polarization element is configured of only inorganic materials, degradation due to heat does not frequently occur, compared to a polarization element using organic materials. However, since a projector obtains a higher brightness year on year, high temperature reliability of an inorganic polarization element which is disposed on a light path thereof becomes a problem due to an increase in temperatures.
As a result of an investigation, it is determined that degradation in an optical property measured when the inorganic polarization element is held at a high temperature, is due to a thermally-oxidized film which grows on a surface of the metallic grid (in general, aluminum is used).
As a technology for protecting the metallic grid, JP-A-2007-17762 discloses that a protecting layer is provided on the metallic grid with an obliquely film-forming method. According to JP-A-2007-17762, a cavity portion is formed by two metallic thin lines which are adjacent to each other, a substrate, and a protecting layer.
However, when the protecting layer is formed with a method disclosed in JP-A-2007-17762, since an end portion of the cavity portion is not covered by the protecting layer, oxygen is supplied to the inside of the cavity portion from the end portion of the cavity portion. Thus, there is a concern that the metallic grid is oxidized in a high-temperature environment.